With rapid development of electronics, communications and computer industries, portable electronic communication equipments such as camcorders, mobile phones, notebook PCs (Personal Computers) or the like have been remarkably developed. Consequently, as a power source for driving these portable electronic communication equipments, the demand for lithium secondary batteries is increasing day by day. In particular, in application of electric vehicles, uninterruptible power supplies, motor tools or artificial satellites, research and development of the lithium secondary batteries as an environmentally friendly power source is lively made inside and outside of the country including Japan, Europe and U.S.A. As commercialization of the lithium secondary batteries is expanded, the lithium secondary batteries are studied in the field of high capacity and safety.
Conventionally, lithium cobalt oxide (LiCoO2) was mainly used as a cathode active material for the lithium secondary batteries, however currently other layered cathode active materials are also used, for example, lithium nickel oxide (Li(Ni—Co—Al)O2), lithium composite metal oxide (Li(Ni—Co—Mn)O2) and so on. In addition, attention is given to a low-costly and highly safe spinel-type lithium manganese oxide (LiMn2O4) and an olivine-type lithium iron phosphate compound (LiFePO4).
However, lithium secondary batteries using the lithium cobalt oxide, lithium nickel oxide or lithium composite metal oxide have excellent basic battery characteristics, but insufficient safety, especially thermal stability and overcharge characteristics. To solve this problem, various safety mechanism has been introduced, for example, a shut-down function of a separator, an additive of an electrolyte, a safety device such as a protection circuit or a PTC (Positive Temperature Coefficient) device, however said mechanism was designed under such conditions that the filling capability of a cathode active material is not too high. Thus, when the filling capability of a cathode active material is increased to meet the demand for high capacity, it causes insufficient operation of said safety mechanism and deterioration of safety.
And, lithium secondary batteries using the spinel-type lithium manganese oxide were once applied to mobile phones, but the batteries did not meet the demand for high energy density in the mobile phone market pursuing advanced function, and thus, had no chance to prove their merits of low cost and high safety. Recently, the spinel-type lithium manganese oxide get attention as a cathode active material for lithium secondary batteries. However, the spinel-type lithium manganese oxide does not meet the demand for high energy density due to its low electrical capacity.
The olivine-type lithium iron phosphate compound has advantages of low cost and high safety, but makes it difficult to expect excellent battery characteristics due to its very low electronic conductivity, and does not meet the demand for high capacity due to its low average operating potential.
Accordingly, various studies have been made to solve the above-mentioned problems, but an effective solution has not been suggested to date.
For example, Japanese Laid-open Patent Publication No. 2001-143705 discloses a cathode active material, in which lithium cobalt oxide and lithium manganese oxide are mixed. However, this prior art simply involves mixing with lithium manganese oxide having excellent safety, and shows an insufficient safety improvement.
And, Japanese Laid-open Patent Publication No. 2002-143708 suggests a cathode active material containing a two-layered lithium nickel composite oxide of different compositions, however this prior art does not fundamentally show sufficient safety improvement after overcharge due to the two-layered lithium nickel composite oxide of different compositions.
Japanese Laid-open Patent Publication No. 2007-012441 discloses a battery comprising a cathode having at least two-layered cathode active material layer so as to improve overcharge characteristics, and suggests the use of olivine-type lithium iron phosphate oxide or spinel-type lithium manganese oxide as a cathode active material layer adjacent to a cathode current collector. The overcharge characteristics improvement is expected, however as it is difficult to form the oxide layers with a thickness below an average particle size, the oxide layers have a thickness of about several μm. Furthermore, the cathode active material of this prior art does not contain a conductive material or a conductive additive, and thus is considered to have insufficient high-current discharge characteristics.
Meanwhile, Japanese Laid-open Patent Publication No. 2006-19229 suggests surface-coating secondary particles of lithium nickel oxide with lithium cobalt zirconium oxide to improve the poor safety of the lithium nickel oxide. However, a wet coating process is used to surface-coat the secondary particles of lithium nickel oxide with the lithium cobalt zirconium oxide, and thus safety is remarkably improved, but productivity is limitative.
Therefore, there is an urgent need for a cathode active material having high safety as well as excellent battery characteristics, and a method of preparing the cathode active material with excellent productivity.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.